Particle track etching method

ABSTRACT

A NEW PARTICLE TRACK ETCHING METHOD IS DISCLOSED WHEREIN A SMOOTHER AND MORE UNIFORM TRACK PROFILE IS OBTAINED BY CONTROLLING THE CHEMICAL CONCENTRATIONS OF THE REAGENTS AND PRODUCTS SO THAT THE ETCHING OCCURS AT A UNIFORM RATE. SPECIFICALLY, THE ETCHANT IS MAINTAINED SATURATED WITH ETCH PRODUCTS THROUGHOUT THE ENTIRE ETCHING PROCEDURE.

June 27, 1972 D. D. PETERSON 3,673,017

PARTICLE TRACK ETCHING METHOD Filed. July 9, 1970 C(GRAMS/ LITER) INVE/V TOR.

04 W0 0. PETERSON,

)Q/ [1 iuw/ HIS ATTORNEY.

United States Patent 3,673,017 PARTICLE TRACK ETCHING METHOD David D. Peterson, Bristol, England, assignor to General Electric Company Filed July 9, 1970, Ser. No. 53,495 Int. Cl. B44c 1/22; C23f 1/00 U.s. c1. 156-7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to particle track etching methods and in particular to an improved method whereby the bore made by the etchant along the path of the particle has a uniform profile as opposed to an irregular profile obtained by present methods.

As is now well known, tracks left by atomic particles in solids can be enlarged for study by the use of an etchant solution. The enlarged bores made by the etchant solution, while suitable for studies relative to such things as the track length and direction, are not as suitable for such uses as filter elements having ultra-small holes as those of the present invention. This is because presently used etchants cause the hole to have a non-uniform profile.

In view of the above, it is therefore an object of the present invention to provide a particle track etching method wherein the bore made by the etchant has a uniform profile.

The foregoing object is achieved wherein the etchant material, from the start of the etchant process to the completion of that process, is a saturated solution; i.e., saturated with the etch product. It has been found that as the etch product concentration is increased there is an abrupt change in the etching rate at the solubility limit of the etch products. By controlling the etching process so that it takes place at the maximum rate over the entire surface of the material being etched, a uniform profile is obtained along the particle track.

A more complete understanding of the present invention may be obtained by considering the following detailed description in conjunction with the attached drawing in which:

FIG. 1 illustrates the general etching process.

FIG. 2 is a plan view of a comparison of the etching process according to the present invention and that of the prior art.

FIG. 3 is a side view of a comparison of the etching process according to the present invention and that of the prior art.

FIG. 4 illustrates curves showing bulk etching rate as a function of etch product concentration.

Referring to FIG. 1 there is shown a block of material containing a particle track 11. The material 10 may comprise any suitable dielectric material such as a plastic film which, for example, may be Lexan polycarbonate film. As a charged particle passes through film 10, it leaves a track of damage 11.

Upon being subjected to an etching solution, the surface of the film 10 is etched away together with region along the track 11. The rate at which the surface of the film 10 is worn away is known as the general etching rate. The rate at which the track is etched away is called the track etching rate. To obtain a track upon etching, the track ice etching rate must be larger than the general etching rate and the ratio of these two determines the shape of the bore left by the etching solution. The configuration of film 10 after being subjected to the etching solution is illustrated by dashed lines 12 and 13. The thickness of the film 10 has been reduced to the point indicated by line 13 and is determined by the general etching rate. The enlarged track portion 12, which may be conical in shape, has its height determined by the track etching rate. The profile of the bore 12 is generally rather irregular when etchants of the prior are are used. It has been discovered that upon saturation of the etchant solution with etch products, the rate of etching abruptly increases to a maximum and remains at the level for as long as the etching solution remains saturated. Utilizing this method, much of the irregularity of profile can be alleviated.

A comparison of the method of the prior art and that of the present invention is illustrated in FIGS. 2 and 3. As shown in FIGS. 2 and 3, the bore 15 resulting from the etchings of the prior art has an irregular profile 14.

In accordance with the present invention in which the etchant solution is presaturated with etchant products, the resulting etching takes place at a maximum and a uniform rate resulting in a bore 17 of uniform profile 16.

FIG. 4 illustrates a set of curves showing the bulk etching rate as a function of etch product concentration for two different etchant temperatures, namely, 40 C. and 55 C. An example of an etchant that may be used for Lexan is a 6.25 N aqueous sodium hydroxide solution containing a 0.05 percent Benax 2A1 surfactant or wetting agent. Benax 2A1 is made by Dow Chemical Company. As can be seen from FIG. 3, an etching solution at 40 C. has a bulk etching rate of about 0.08 micron per hour until the etch product solubility limit is reached, whereupon the bulk etching rate abruptly changes to about 0.17 micron per hour. Similarly, the bulk etching rate with a 55 C. etchant gradually increases until the etch product solubility limit is reached whereupon it can abruptly increases to a stable value. Thus, by utilizing an etchant that is saturated with etch products, a smooth bore along the particle track is obtained.

Having thus described the invention, it will be apparent to those skilled in the art that modifications may be made within the scope of the present invention. For example, the method of the present invention may be used in other areas in which materials are etched, as in the semiconductor fabrication art.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An etching method for etching a material with an etching solution producing etch products comprising the steps of:

presaturating said etching solution with said etch products,

subjecting said material to be etched to said presaturated etching solution, and

maintaining said etching solution saturated with said etch products during the etching of said material.

2. A method for etching a material containing particle tracks with an etching solution producing etch products comprising the steps of:

presaturating said etching solution with said etch products,

subjecting said material containing particle tracks to said presaturated etching solution, and

maintaining said etching solution saturated with said etch products during the etching of said material.

3. A process as set forth in claim 2 wherein said etching solution comprises an aqueous hydroxide solution and a surfactant, and said material containing particle tracks comprises a polycarbonate film.

4. A particle track etching process comprising. the 1 H --References-Cited P ir v p t Q 1 t k t UNITED STATES PATENTS SH 16C 111g a ma erla C011 aimng'par 1C6 rac S 0 an 3,373,683 3/1968 Alter 25083 X i t f 2 5? f i -g fg g F5 ifffi: 5 3,415,993 12/1968 Fleischer 6123.1 156-2 X mam ammg a 6 c 1g m 3,418,472 12/1968 Evens 250 s3 predetermined level, whereby the etching of the rnate- 9 will produce a uniformly dimensioned etched particle ROBERT BURNETT, Primary Examiner;

track. 5. A particle track etching process as set forth in 10 Asslstgnt 39m claim 4 wherein said predetermined level is the. maxi US Cl, X,R v 3 mum etching rate. r 250-.-.83'C D 

